Document background drawing with clips — use push/pop, not op.Record macro

doc/layout_rendering.md

@@ -627,6 +627,33 @@ func (r *Renderer) drawBottomBar(gtx layout.Context, bb BottomBar, constraints W
 - **Don't mix Dp and Px**: The compiler will catch it. Use explicit conversions.
 - **Don't pass pixels as Dp to EditorLayout**: `main.go` must convert `app.ConfigEvent` pixels to Dp before passing to the logic layer.
 
+## 9.6 Background Drawing with Clips
+
+When drawing backgrounds (rectangles with solid colors), use `clip.Rect{...}.Push(gtx.Ops)` followed immediately by `clip.Pop()`. **Never use `op.Record`/`m.Stop()` macros for simple background clips** — the macro leaves the clip on the stack, preventing subsequent elements from drawing.
+
+**Correct pattern**:
+```go
+bgClip := clip.Rect{
+    Min: image.Point{X: pxMinX, Y: pxMinY},
+    Max: image.Point{X: pxMaxX, Y: pxMaxY},
+}.Push(gtx.Ops)
+paint.ColorOp{Color: bgColor}.Add(gtx.Ops)
+paint.PaintOp{}.Add(gtx.Ops)
+bgClip.Pop()  // Immediately pop — clip stack must be clean
+```
+
+**Wrong pattern** (leaves clip on stack):
+```go
+m := op.Record(gtx.Ops)
+clip.Rect{...}.Op().Push(gtx.Ops)  // Push inside macro
+paint.ColorOp{...}.Add(gtx.Ops)
+paint.PaintOp{}.Add(gtx.Ops)
+call := m.Stop()
+call.Add(gtx.Ops)  // When replayed, clip is pushed but never popped!
+```
+
+**Why this matters**: When the StatusBar draws its background with a macro, the clip inside is pushed during macro replay but never popped. The BottomBar then draws while that stale clip is still active, causing it to be clipped out of visibility.
+
 ## 10. Summary
 
 - **Always** use `fixed.I(gtx.Sp(size))` for `PxPerEm`